Geometrical spin frustration in Pr₅Ni₂Si₃ composed of triangular crystal lattices

We have studied the transport, magnetic and thermal properties of Pr ₅Ni₂Si₃ with complex triangular lattices under various magnetic fields. The ferromagnetic transitions in the basal plane were observed at T_C1 = 52K and T_C2 = 65K. A decrease in magnetization below 30K and the reduced paramagnetic Curie temperature θ_P are indicative of the development of antiferromagnetic correlation. These features are well understood by the frustration effect of the magnetic moments of Pr ions which constitute the triangular structural unit. The frustration caused the rapid rise of electrical resistivity below 30K and an enormous entropy in low-temperature regions. The antiferromagnetic correlation acting between the frustrated Pr ions never causes any long-range order down to 0.6K.